Yes, You do have to use it right away. It begins to dry as soon as it's
mixed. I did most of the mixing instead of letting students do it. They
waste so much and the mixture is toxic. did not want to take chances.
i found that a disposable piece of matt board and plastic measuring
spoon worked well and I didn't have to worry about cleaning them.
To mix:
Use a piece of matt board to mix on and a small stick like scrap to mix
the 2 equally measured parts together. Put on your mixing board
separately. Instead of mixing like stirring a spoon, use the side of the
matt board scrap to scrape and mix like folding into a cake mix. the
fewer air bubbled you mix in the better. Air bubbles will stay trapped
in the mixture unless you use a toothpick to poke into them. Sometimes
I've used a hair dryer to even out the mixture and get rid of air bubbles.

Another concern is to elevate the piece you're working on. Place on
a trivet smaller than the outside perimeter of the finished piece.
Envioro Tex will flow to the edges of your piece if not contained and it
will stick permanently to the surface you have it placed on. By
elevating it slightly, you avoid that problem. I've also used masking
tape to make a "wall" around the piece I'm working on, though with
jewelry this was a problem.

Wish I'd saved a few pieces to send to anyone that would like to try
this process.
Hope this helps:
Gelato